One AED affected in this way is lamotrigine. Previous studies have shown that lamotrigine levels in the blood drop markedly during pregnancy, and that for many women, seizure frequency goes up. This is because lamotrigine is being processed by the body more quickly than usual, so each dose has a smaller effect than usual.
Researchers from Emory University in Atlanta, Georgia, USA, ran a study to find out how much lamotrigine levels change during pregnancy, and how this affects pregnant women's susceptibility to seizures.
Professor Page Pennell and her colleagues followed a group of 53 women with epilepsy taking lamotrigine throughout their pregnancy. They reviewed the seizure and medication diaries of each mother-to-be once every one-to-three months, and took blood samples at the same time, which they analysed for lamotrigine levels.
They found, on average, an increase in the amount of lamotrigine being cleared by the body (and therefore a decrease in the amount of lamotrigine available for suppressing seizures), and that this effect was greatest in the third trimester of pregnancy.
They also found that the greater the increase in lamotrigine clearance, the greater the chance that the woman experienced a rise in seizure frequency; this was particularly true in the second trimester of pregnancy. However not every woman in the study experienced an increase in seizures during their pregnancy: 33% reported a decrease, and 28% no change, though 39% did report an increase.
Interestingly, the rate of clearance of lamotrigine appeared to be partly dependent on ethnicity: the researchers reported that white women showed higher rates of clearance than black women. It's possible that the rate of metabolism of these drugs has some genetic basis.
These data will be very useful to doctors who treat women with epilepsy taking lamotrigine, as they give detailed information about how to adjust lamotrigine dose during a pregnancy. It's hoped that these results will help improve the health of both mothers and babies.